CD34 is a highly glycosylated transmembrane protein strongly expressed on hematopoietic stem/progenitor cells (HSPCs); despite its importance as a marker of HSPCs, its function is still poorly understood, even if a role in cell adhesion has been demonstrated. In order to characterize the function of CD34 antigen in human HSPCs, we evaluated by small interfering RNAs (siRNAs) mediated gene silencing the role of CD34 antigen in HSPCs differentiation. By using the Nucleofection Amaxa technology for siRNA transfection in HSPCs, we obtained a rapid and effective down-regulation of the CD34 antigen. In this paper, we have demonstrated that CD34 silencing in HSPCs enhances their granulocyte and megakaryocyte differentiation and reduces erythroid maturation as shown by clonogenic assay, morphological analysis and expression of differentiation markers. In agreement with these results, the gene expression profile of HSPCs-silenced cells reveals the up-regulation of genes involved in granulocyte and megakaryocyte commitment and the down-regulation of erythroid genes. These data indicate that CD34 transmembrane protein promotes the differentiation of CD34+ hematopoietic progenitors towards the erythroid lineage at the expense of granulocyte and megakaryocyte ones.